admin: Fix help on error

The admin shell used to output a command summary in case of syntax errors,
however recent refactoring broke this when the admin shell was ported to
CellStub (CellStub assumes that dCache uses CacheException, while much code
that comes from cells relies on CommandException).

This patch resolves the issue.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/6865/